1. Field of the Invention
The present invention generally relates to a color picture tube for displaying colored pictures and, more particularly, to a color cathode ray tube of a type having a shadow mask.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings illustrates, in schematic longitudinal sectional representation, the exemplary prior art cathode ray tube of a type having a shadow mask. The color cathode ray tube shown therein comprises a highly evacuated envelope 1 including a funnel section closed at one end by a faceplate 11 and at the opposite end continued to a generally cylindrical neck section. The neck section has an electron gun assembly 2 accommodated therein for emitting three electron beams S. The faceplate 11 has an inner surface deposited with a predetermined pattern of primary color elemental phosphor deposits, for example, triads of red, blue and green phosphor dots, thereby to form a phosphor deposited screen 1a. An apertured shadow mask 4 is supported within the envelope 1 in a well known manner generally in parallel relationship with the phoshor deposited screen 1a and spaced a predetermined distance inwardly from the phosphor deposited screen 1a. The envelope 1 has a deflection yoke assembly 3 mounted thereon at the boundary between the neck section and the funnel section for developing a horizontal deflection magnetic field and a vertical deflection magnetic field in a well known manner.
In this construction, the three electron beams S emanating from the electron gun assembly 2 travel towards the phosphor deposited screen 1a. During the travel of the electron beams S towards the phosphor deposited screen 1a, the electron beams are deflected under the influence of the horizontal deflection magnetic field so as to scan the phosphor deposited screen 1a generally horizontally, that is, along the horizontal scanning lines, and also under the influence of the vertical deflection magnetic field so as to retrace the phosphor deposited screen 1a generally vertically. The vertical movement of the electron beams S takes place after the electron beams S have scanned the phosphor deposited screen 1a horizontally from top to bottom.
The electron beams S having passed through the deflection magnetic field pass through the apertures in the shadow mask 4 and then impinge upon the phosphor deposited screen 1a, allowing the triads of the primary elemental color phosphor dots, which are stricken by the electron beams S, to emit light. Actual image reproduction is accomplished by scanning the electron beams S across the phosphor deposited screen 1a while the electron beams S passing through the apertures in the shadow mask 4 successively impinge upon the triads of the primary color elemental phosphor dots.
FIG. 2 illustrates a portion of the shadow mask 4 used in the prior art cathode ray tube on an enlarged scale for the purpose of showing the details thereof. Let it be assumed that the widthwise direction of the phosphor deposited screen 1a parallel to the horizontal scanning lines is represented by an X-axis and the heightwise direction of the same screen la perpendicular to the widthwise direction thereof is represented by a Y-axis, with the point of origin of the X-Y coordinate system being occupied by the center of the phosphor deposited screen 1a that is aligned with the longitudinal axis (or Z-axis) of the envelope of the cathode ray tube. As shown, the shadow mask 4 has a plurality of vertically extending parallel rows of slots 4a of equal length, each of said rows extending parallel to the Y-axis direction and each of said slots 4a having a longitudinal axis also lying parallel to the Y-axis. When the pitch between each neighboring slots 4a in each row is expressed by Pv, the slots 4a in one of the rows and the slots 4a in the next adjacent row are offset vertically with respect to each other by a distance equal to half the slot pitch Pv. In other words, the slots 4a in the respective rows are alternately staggered relative to each other.
Since each bridge portion 4b of the shadow mask 4 delimited by the neighboring slots 4a in each row blocks the passage of the electron beams S traveling towards the phosphor deposited screen 1a, it is observed that, during the operation of the color cathode ray tube, rows of shadows, spaced a distance equal to half the slot pitch Pv, of the bridge portion 4b are cast horizontally upon the phosphor deposited screen 1a, thereby forming a pattern of bright and dark fringes occasioned by the bridge portions 4b.
On the other hand, it is well known that the number of the horizontal scanning lines is fixed at 525 lines according to the NTSC television system and 625 lines according to the PAL television system. It is also well known that the electron beams S have their own size which is smaller than the distance between the neighboring horizontal scanning lines. Accordingly, a shadow is observed between the neighboring scanning lines which forms a pattern of bright and dark fringes occasioned by the electron beams S.
Therefore, when the shadows occasioned by the bridge portions 4b of the shadow mask 4 and the shadow occasioned by the electron beams S interfere with each other, the result is the apperance of Moire patterns in the reproduced pictures.
In order to minimize the appearance of the Moire patterns in the pictures being reproduced on the screen of the color cathode ray tube, the slot pitch Pv is carefully selected. The selection of the slot pitch Pv for the purpose of minimizing the appearance of the Moire patterns is generally carried out by the following manner. Assuming that, as shown in FIG. 3 of the accompanying drawing which illustrates a partial cross-section of the faceplate 11 of the color cathode ray tube together with the shadow mask 4 in relation to the center of deflection indicated by 10, the distance equal to half the slot pitch Pv, which is hereinafter referred to as "half slot pitch", is expressed by Pa, that is, Pv/2=Pa; the distance between the neighboring horizontal scanning line as measured on the shadow mask 4 in the vertical direction is expressed by Ps; and the recurrent interval of the Moire patterns (hereinafter referred to as "Moire pitch") is expressed by Pm, the following relationship can be established. EQU Pm(m, n)=.vertline.(2Ps.multidot.Pa)/(2mPs-nPa).vertline. (1)
wherein m and n represent an integer. The result of experiment has shown that, in the case (a) where m and n are 1 and 3, respectively, or the case (b) where m and n are 1 and 4, respectively, or the case (c) where m and n are 1 and 5, respectively, the Moire patterns tend to become prominent. The relationship between the normalized Moire pitch (which is represented by the recurrent interval Pm of the Moire patterns divided by the effective diameter as measured in the vertical direction) and the normalized half slot pitch (which is represented by the half slot pitch Pa divided by the effective diameter as measured in the vertical direction), which is found in the NTSC television system, is shown in FIG. 4. It is to be noted that the term "effective diameter as measured in the vertical direction" referred to above and hereinafter is intended to mean the length of that portion of the shadow mask where the slots are formed as taken in the Y-axis. In the case of the 27-inch, 110.degree. deflection color cathode ray tube, the Moire pattern can be minimized when the normalized distance is 1.28.times.10.sup.-3, in which case the slot pitch Pv gives 0.91 mm. The use of the increased number for the half slot pitch Pa in the equation (1) above is effective to increase the recurrent interval Pm and consequently to minimize the Moire patterns. However, since as is well known to those skilled in the art the shadow mask is so deformed as to assume a generally spherical shape, the slot pitch Pv is more or less smaller than 1.5 mm. When the slot pitch Pv is smaller than 1.5 mm as shown in FIG. 3, that is, when the normalized half slot pitch is smaller than 2.1.times.10.sup.-3, complete removal of the appearance of the Moire patterns in the reproduced pictures is not possible. Although the appearance of the Moire patterns in the reproduced pictures can be reduced if the width B of each bridge portion 4b as indicated in FIG. 3 is reduced because the reduction in bridge width B corresponds to the use of the increased slot pitch Pv, the problem associated with manufacturing of the shadow mask necessitates the employment of the bridge width B within a predetermined range regardless of the particular value for the slot pitch Pv, particularly 0.1 mm.ltoreq.B.ltoreq.0.15 mm. The size of the shadow cast upon the phosphor deposited screen 1a under the influence of the bridge width B tends to increase in proportion to the increase of the deflection angle and in inverse proportion to the curvature of the shadow mask 4 (or in proportion to the radius of curvature thereof).
Also, the width of each horizontal scanning line, as will be described later, tends to be lessened with improvement of the focusing of the electronic lens used in the color cathode ray tube. Particularly in the case of the color cathode ray tube wherein the sophisticated electron gun assembly is employed which is effective to permit the image to be accurately focused substantially all over the phosphor deposited screen by applying a modulated voltage synchronized with the deflection current to the focusing electrodes used in the electron gun assembly, bright and dark stripes of the scanning lines tend to be prominent all over the phosphor deposited screen and the pattern of distribution of the Moire pitches attributable to the interference thereof with the bright and dark fringes resulting from the bridge portions 4b varies from place to place on the phosphor deposited screen. Therefore, with such color cathode ray tube using the sophisticated electron gun assembly, the use of the constant slot pitch Pv tends to result in the considerable appearance of the Moire pattern.
The inventor of the present invention is aware that anyone of the U.S. Pat. No. 3,973,159, No. 4,210,842 and No. 4,326,147, issued Aug. 3, 1976, July 1, 1980, and Apr. 20, 1982, respectively, discloses a technique for suppressing the appearance of the Moire patterns in the reproduced pictures by varying the half slot pitch in the Y-axis direction in a predetermined relation. However, it has been found that none of the prior art techniques is satisfactory.
Summarizing the above, since in the prior art color cathode ray tube of the type using the apertured shadow mask, the slot pitch are uniform all over the entire surface of the shadow mask, minimization of the appearance of the Moire patterns anywhere on the phosphor deposited screen has been difficult to achieve.